#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
#include<math.h>
int main()
{
	int n = 0;
	char arr[1005] = { 0 };
	scanf("%d", &n);
	scanf("%s", &arr);
	int length = strlen(arr);
	int i = 0, cnt = 0;
	for (i = 0; i < length; i++)
	{
		while (1)
		{
			if (arr[i + 1] == arr[i])
			{
				cnt++;
				goto back;
			}
			else
				cnt = 0;
				break;
		}
		back:
			;
		if (cnt+1 == n)
		{
			printf("%c", arr[i]);
			return 0;
		}
	
	}
	printf("No");
	return 0;
}
